GaN-based light emitting diodes (LEDs) have been employed and developed for about 10 years or more. The GaN-based LEDs considerably have changed LED technologies and are currently used for various applications, such as full-color LED display devices, LED traffic lights and white LEDs. Recently, it has been expected that high-efficiency white LEDs will substitute for fluorescent lamps. Particularly, the efficiency of the white LEDs has reached the level similar to that of typical fluorescent lamps.
In general, an LED emits light by forward current and requires the supply of DC. Hence, if the LED is connected directly to an AC power source, it is repeatedly turned on/off depending on the direction of current. As a result, there are problems in that the LED does not continuously emit light and is easily broken by reverse current.
To solve such a problem, an LED capable of being connected directly to a high-voltage AC power source is disclosed in PCT Patent Publication No. WO 2004/023568(A1), entitled “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS” by SAKAI et al.
According to PCT Patent Publication No. WO 2004/023568(A1), LEDs (i.e., light emitting cells) are two-dimensionally connected in series on a single insulative substrate such as a sapphire substrate to form LED arrays. Such two LED arrays are connected to each other in reverse parallel on the sapphire substrate.
A GaN-based LED is generally formed by growing epitaxial layers on a substrate such as sapphire and comprises an N-type semiconductor layer, a P-type semiconductor layer and an active layer interposed therebetween. Meanwhile, an N-type electrode is formed on the N-type semiconductor layer, and a P-type electrode is formed on the P-type semiconductor layer. The LED is electrically connected to an external power source through the electrodes, thereby being driven. At this time, a current flows from the P-type electrode into the N-type electrode via the semiconductor layers.
Since the P-type semiconductor layer generally has high specific resistivity, there is a problem in that the current cannot be uniformly distributed but is concentrated on a portion at which the P-type electrode is formed. The current concentration leads to the reduction of a light emitting area, and therefore, the light emission efficiency is lowered. In order to solve such a problem, the technology for distributing current by forming a transparent electrode layer with a low specific resistivity on a P-type semiconductor layer is used. Since the current introduced from the P-type electrode is distributed in the transparent electrode layer and flows into the P-type semiconductor layer, the light emitting area of the LED can be expanded.
However, since the transparent electrode layer absorbs light, the thickness of the transparent electrode is restricted, and therefore, there is a limit in the current distribution. Particularly, a large-sized LED having an area of about 1 mm2 or more used for high power has a limit in the current distribution using the transparent electrode layer.
Meanwhile, the current flows through semiconductor layers and exits through the N-type electrode. Thus, the current is concentrated on a portion of the N-type semiconductor layer on which the N-type electrode is formed, which means that the current flowing in the semiconductor layers is concentrated near the portion on which the N-type electrode is formed. Therefore, it is required to develop an LED capable of improving current concentration in an N-type semiconductor layer.
In the meantime, P-type and N-type electrodes formed in an LED generally block light emitted from the LED. Therefore, it is necessary to improve the structure of the P-type and N-type electrodes, which can enhance the light emission efficiency of the LED.
An object of the present invention is to provide an AC LED having an electrode structure in which a current flowing in operation of the LED can be uniformly distributed and the light efficiency can be enhanced.
According to an aspect of the present invention for achieving the objects, there is provided an AC LED, which comprises a substrate, and at least one serial array having a plurality of light emitting cells connected in series on the substrate. Each of the light emitting cells comprises a lower semiconductor layer consisting of a first conductive compound semiconductor layer formed on top of the substrate, an upper semiconductor layer consisting of a second conductive compound semiconductor layer formed on top of the lower semiconductor layer, an active layer interposed between the lower and upper semiconductor layers, a lower electrode formed on the lower semiconductor layer exposed at a first corner of the substrate, an upper electrode layer formed on the upper semiconductor layer, and an upper electrode pad formed on the upper electrode layer exposed at a second corner of the substrate. The upper electrode pad and the lower electrode are respectively disposed at the corners diagonally opposite to each other; and the respective light emitting cells are arranged so that the upper electrode pad and the lower electrode of one of the light emitting cells are symmetric with respect to those of adjacent another of the light emitting cells.
The serial array may comprise two serial arrays connected in reverse parallel to each other.
The two serial arrays connected in reverse parallel to each other, at least one of the light emitting cells positioned in any one of the serial arrays is electrically connected to the corresponding one of the light emitting cells positioned in the other serial array. Accordingly, it is possible to prevent overvoltage caused by a reverse voltage from being applied to a specific serial array during the operation of the AC LED.
The upper electrode layer may be a transparent electrode layer.
The transparent electrode layer may be formed of indium tin oxide (ITO) or Ni/Au.
The upper electrode pad may be formed of at least one selected from Ni, Cr, Pd, Pt, W and Al.
The upper electrode pad may comprise at least one layer or alloy layer.
The lower electrode may be formed of at least one selected from Ni, Cr, Pd, Pt, W and Al.
The lower electrode may comprise at least one layer or alloy layer.
The active layer may include a single quantum well or multiple quantum well structure having an InxGa1-xN (0<x<1) well layer and an InxGa1-xN (0≦x<1) barrier layer.
The InxGa1-xN (0<x<1) well layer may have more In content than the InxGa1-xN (0≦x<1) barrier layer.
The second conductive compound semiconductor layer may include AlxGa1-xN (0≦x<1).
According to another aspect of the present invention, there is provided an AC LED, which comprises: two or more single chips electrically connected in series. Each of the single chips comprises a substrate, and at least one serial array having a plurality of light emitting cells connected in series on the substrate. Each of the light emitting cells comprises a lower semiconductor layer consisting of a first conductive compound semiconductor layer formed on top of the substrate, an upper semiconductor layer consisting of a second conductive compound semiconductor layer formed on top of the lower semiconductor layer, an active layer interposed between the lower and upper semiconductor layers, a lower electrode formed on the lower semiconductor layer exposed at a first corner of the substrate, an upper electrode layer formed on the upper semiconductor layer, and an upper electrode pad formed on the upper electrode layer exposed at a second corner of the substrate. The upper electrode pad and the lower electrode are respectively disposed at the corners diagonally opposite to each other; and the respective light emitting cells are arranged so that the upper electrode pad and the lower electrode of one of the light emitting cells are symmetric with respect to those of adjacent another of the light emitting cells.
The serial array may comprise two serial arrays connected in reverse parallel to each other.
The two or more single chips may be mounted on respective packages and are connected in series by a bonding wire.
The two or more single chips may be mounted on respective packages, and the packages are connected in series.